In recent years, with higher density and higher integration of electronic components and semiconductor devices such as integrated circuits (ICs), large scale integrated circuits (LSIs), very large scale integrated circuits (VLSIs) and the like, a method of mounting thereof has been changed from insertion mounting method to a surface mounting method. With that, lead frames with an increased number of pins and lead with narrower pitch intervals have been in demand, and surface mounting type QFPs (Quad Flat Packages) and the like capable of coping with small size and lightweight have been used for various semiconductor devices. Encapsulating of the semiconductor device is mainly conducted using an epoxy resin composition from the viewpoint of excellent balance of productivity, cost, reliability and the like.
In the past, for the purpose of imparting flame retardance, a bromine-containing epoxy resin and an antimony oxide have been generally used for an epoxy resin composition for encapsulating a semiconductor. However, recently, there is a growing move to limit the use of a halogenated compound which might generate a dioxin analogue compound or a highly toxic antimony compound from the viewpoint of environmental protection. As an alternative flame retardant of a brominated epoxy resin or an antimony compound, metal oxides such as aluminum hydroxide, magnesium hydroxide and the like have been used. However, these flame retardants cause deterioration of flowability due to an increase in the molten resin viscosity, or deterioration of solder resistance in some cases.
From the above-stated circumstances, as an epoxy resin composition for encapsulating a semiconductor capable of achieving excellent flame retardance without the need of adding a flame retardance-imparting agent, there has been proposed an epoxy resin composition using a phenol aralkyl type epoxy resin having a biphenylene skeleton (for example, Patent Documents 1 and 2). These epoxy resin compositions have advantages of flame retardance, low moisture absorption, low elastic modulus when being heated and high adhesiveness, whereas such resins do not exhibit enough flowability for encapsulating a thinned and miniaturized semiconductor device.    Patent Document 1: JP-A-2004-203911    Patent Document 2: JP-A-2004-155841